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Monsoon in the desert?

Monsoon storm clouds building up over the desert. Source: http://arizonaexperience.org/land/arizonas-monsoon-season

I made a reference yesterday that “we are headed into the season of monsoon storms”, and one of my readers made a comment that he did not associated “monsoon” with “desert”. But he did his homework and added a link to an excellent article that explains the phenomenon. Several years ago a granddaughter from Wyoming mentioned a monsoon reference from my blog to a high school teacher, who then took her to task saying “there are no monsoons in the US”, or similar. Once I provided her with the explanation, she passed it along to the teacher, who then apologized. I think the link provided in the comment to yesterday’s story explains the monsoon concept as well as I could, so I will quote from it here.

“Though the word monsoon is often used to refer to a single thunderstorm, it is actually the name of the large scale weather pattern. The word derives from the word mausin, Arabic for “season” or “wind shift.” It is now used to refer to a seasonal wind shift and the precipitation produced as a result. Parts of Africa, Asia, North America, Australia, and Europe experience a monsoon season… Arizona winds usually come from the west, but shift to a southeasterly wind in the summer, bringing moisture, most often from the Gulfs of Mexico and California. The wind shift and increase in moisture combine with the surface low pressure from the desert heat to produce storms in a cycle of “bursts” (heavy rainfall) and “breaks” (reduced rainfall). Before the rain, the wind shift can trigger dust storms known as haboobs [more about haboobs another time], which appear as loose swirling walls of dust several hundred feet high.” http://arizonaexperience.org/land/arizonas-monsoon-season

An immediate effect of the monsoon is the buildup of clouds. During May and June we saw mostly clear blue skies, day after day. Now in July we start to see afternoon clouds in the sky, mostly over the McDowell Mountains to the east and the Mogollan Rim to the north. As the season develops further those clouds will be awesome towering cumulus banks, reaching 20,000-30,000 feet high. Especially with those clouds over the McDowells, we’ll see lots of lightning flashing cloud-to-cloud and ground-to-cloud. Those produce some vivid light displays during summer evenings.

Another aspect of monsoon is, of course, rainfall. We get about 35% of our annual rainfall during monsoon, on average. That’s 2½-3½ inches. That rainfall comes in bursts – heavy storms for brief periods. A danger during monsoon is flooding in the normally-dry washes, and this can sneak up on us. Heavy rainfall in the hills and mountains ten miles away can result in flash floods in dry washes in the valleys where no rain has fallen. Especially in less-developed areas roads cross washes with “dip crossings” instead of culverts; the wash simply floods over the roadway.